The invention relates to acoustic logging apparatuses and methods. Such apparatuses and methods are sometimes also referred to using the adjective “sonic”, although the term “acoustic” is preferentially used herein for convenience.
As is well known, prospecting for minerals of commercial or other value (including but not limited to hydrocarbons in liquid or gaseous form; water e.g. in aquifers; and various solids used e.g. as fuels, ores or in manufacturing) is economically an extremely important activity. For various reasons those wishing to extract such minerals from below the surface of the ground or the floor of an ocean need to acquire as much information as possible about both the potential commercial worth of the minerals in a geological formation and also any difficulties that may arise in the extraction of the minerals to surface locations at which they may be used.
For this reason over many decades techniques of logging of subterranean formations have developed for the purpose of establishing, with as much accuracy as possible, information as outlined above both before mineral extraction activities commence and also, increasingly frequently, while they are taking place.
Broadly stated, logging involves inserting a logging tool including an energy generating section sometimes called a “sonde” into a borehole or other feature penetrating a formation under investigation; and using the sonde to energise the material of the rock, etc, surrounding the borehole in some way. The sonde or another tool associated with it that is capable of detecting energy is intended then to receive emitted energy that has passed through the various components in the rock before being recorded by the logging tool.
Such passage of the energy alters its character. Knowledge of the attributes of the emitted energy and that detected after passage through the rock may reveal considerable information about the chemistry, concentration, quantity and a host of other characteristics of minerals in the vicinity of the borehole, as well as geological aspects that influence the ease with which the target mineral material may be extracted to a surface location.
Logging techniques are employed throughout the mining industry; in the identification of subterranean sources of water; and also in particular in the oil and gas industries. The invention is of benefit in logging activities potentially in all such kinds of investigation, and especially in the logging of reserves of oil and gas.
In the logging of oil and gas fields (including fields combined with rock types such as shales or coal beds) specific problems can arise. Broadly stated this is because it is necessary to consider a geological formation that typically is porous and that contains a hydrocarbon-containing fluid such as oil or gas or (commonly) a mixture of fluids only one component of which is of commercial value.
Moreover for example stratification of rock, micro-cracking and the effects of applied stress often give rise to anisotropic characteristics that adversely affect the ease with which log data can be interpreted. Such phenomena are widely reported in the technical literature pertaining to logging activity.
These factors lead to various complications associated with determining physical and chemical attributes of the oil or gas field in question. In consequence a wide variety of logging methods has been developed over the years. The logging techniques exploit physical and chemical properties of a formation usually through the use of an elongate logging tool or sonde that is lowered into a borehole (that typically is, but need not be, a wellbore) formed in the formation by drilling. Sometimes the term “logging toolstring” is used to refer to logging tools made up of a series of elongate subcomponents that are joined end to end to create a tool providing a chosen combination of performance features.
The tool or toolstring typically is conveyed to a downhole location (the terms “downhole” and “uphole” being familiar to the person of skill in the art) suspended on wireline or supported on drillpipe. The nature and operation of wireline and drillpipe for these purposes, and the use of wireline for the transmission of log data to a surface location, are well known in the data logging arts.
Typically, as noted, once the tool reaches a location at which logging is to occur it sends energy into the formation on demand and detects the energy returned to it that has been altered in some way by the formation. The nature of any such alteration can be processed into electrical signals that are then used to generate logs (i.e. graphical or tabular representations containing much data about the formation in question).
Conventionally logging takes place by conveying the logging tool to a downhole location and then withdrawing it upwardly while logging takes place. However it is also known to carry out logging activity while a borehole is being drilled. Such techniques are known as “logging while drilling” or “LWD” activities. The invention relates generally to conventional logging sequences and LWD methods.
One form of logging technique is known as acoustic logging. As the name implies, in acoustic logging, acoustic (i.e. mechanical fluid wave) energy is generated by the logging tool and transmitted into the rock surrounding a borehole. The energy returned to the tool after passing through the rock is detected by one or more acoustic energy detectors sometimes referred to as hydrophones.
In a typical acoustic dipole logging tool the sonde section is hollow and contains a pair of centrally mounted bender bars secured seriatim in the tool. The bender bars include piezoelectric elements that may be energized under the influence of a control circuit to cause them to deflect in opposite directions and thereby generate a dipole wave (or pressure pattern) in fluid surrounding the tool in the borehole. Depending on the relative speeds of propagation of the wave in the rock and the fluid the wave energy may be caused to penetrate the rock and thereby travel through it before being detected by a set of (typically) four mutually orthogonal hydrophones that are secured in the logging tool, in use vertically above the bender bar.
A sonde section used in this manner includes apertures on opposite sides of the housing, in register with the bender bars. These permit energy resulting from deflection of the bender bars to create the desired pressure pattern in the borehole fluid surrounding the sonde, and therefore generate the required flexural wave energy in the borehole-formation system. As is known to the person of skill in the art, a dipole wave (or pressure pattern) in a borehole gives rise to a dipolar displacement which is flexural in nature.
A sleeve typically surrounds the housing in a fluid-tight manner. The hollow interior of the sonde in addition to mounting the bender bars contains an oil. The combination of the movement of the bender bars and the characteristics of the oil cause propagation of the wave in the fluid. The sleeve is therefore sufficiently flexible (or flexibly secured) and light as to be transmissive of the energy of the wave. The sleeve may be made from a range of materials that do not attenuate the generated wave.
The constructional principles of a prior art acoustic logging transmitter 10 are shown in a highly schematic, three dimensional view in FIG. 1.
In FIG. 1 a piezoelectric bender bar 11 is shown captured between two relatively rigid end masses 12, 13 that may be for example end caps retained within a housing defining the transmitter 10.
The bender bar 11 adopts an elongate, rectangular form that at each end is secured to a respective said end mass 12, 13 via a hinge represented schematically by numeral 14. The bender bar extends longitudinally along the length of the transmitter, that typically is a so-called “sub” or a section of a logging tool that is assembled from a series of subs that are secured together end to end, as is well known in the logging art.
Each hinge 14 may be formed by a pivoted linkage arrangement or, for instance, a flexible part of the bender bar or a flexible intermediate element joining the bender bar 11 and the end masses 12, 13. The purpose of the hinges 14 is to permit the bender bar 11 to flex relative to the end masses, and also accommodate its characteristic shortening, when the bender bar is energized through the application of an electrical voltage generated in a driver circuit.
The bender bar may be formed as e.g. two rectangular layers or plates of piezoelectric material respectively extending along and bonded to either side of a central rectangular neutral plate. This is sometimes referred to as a trilaminar construction. The neutral plate is capable of flexing to adapt to flexed configurations of the piezoelectric plates. Its ends extend beyond the ends of the piezoelectric plates, and are secured to the hinges 14.
Connections permit the application of voltages generated by the driver circuit to the piezoelectric plates. As is well known the application of a voltage to a plate of piezoelectric material changes its dimensions while the voltage is applied. By applying differential voltages to the respective piezoelectric plates on opposite sides of the neutral plate one may cause the entire bender bar to flex along its length and thereby adopt a curved configuration.
The result of this arrangement is that on the application of appropriate voltages the bender bar flexes relative to the end masses 12, 13 with the hinges 14 accommodating both the lateral movement and the reduction in the distance between the ends of the bender bar occasioned by its flexing. The principal movement of the bender bar is signified by the double-headed arrow in FIG. 1.
The transmitter 10 normally would include an outer cylindrical sleeve encircling the bender bar arrangement. The sleeve, that is omitted from FIG. 1 because of the schematic nature of this illustration, is filled with an oil. Structural parts of the transmitter include one or more windows that are transmissive to pressure waves and pulses while the sleeve prevents egress of the oil. The controlled application of voltages to the piezoelectric plates forming the bender bar therefore creates pressure patterns in the oil that may be transmitted to borehole fluid surrounding the transmitter 10 via the windows. The driver circuit may be programmed to cause the generation of desired sequences of pressure pulses in the borehole fluid.
In a practical transmitter there would exist more than one of the bender bars, arranged inside the housing in a manner that gives rise to chosen pressure patterns in the borehole fluid. The housing may include plural numbers of appropriately designed and located windows that permit the propagation of the pressure patterns in the borehole fluid.
The driver circuit includes or is connected to a programmable device that typically causes one, or in some constructions two, bender bars inside the housing simultaneously to bend in opposite directions. As a result for example, a positive pressure pulse emanates via one of the windows in the housing and a negative pressure emanates via an oppositely located window in turn resulting in a characteristic dipole directivity. In consequence a symmetrical dipole pressure pulse may be generated. Oscillatory activation of two bender bars in the transmitter in this manner gives rise to a desired flexural wave that can be used to energise a formation surrounding the transmitter for logging purposes.
In practical logging tools two of the transmitter sections 10 may be joined seriatim in a logging toolstring. The windows of the respective transmitters are orientated orthogonally to one another so that two orthogonal dipoles may be generated.
Such an approach is useful for example when the formation exhibits shear anisotropy, a well-known phenomenon in which shear waves become polarized into components propagating respectively parallel to and perpendicular to the fast and slow velocity principle axes, e.g. the prevailing lines of fracture in the rock. The wave component propagating parallel to the fracture direction has a higher velocity in the rock than the wave component propagating perpendicular to the fracture.
It is sometimes desirable to co-locate the transmitters at a common depth in the borehole instead of at different, albeit adjacent, depths as described. This eliminates some problems associated with depth-matching the signals generated at the hydrophones (and that are used to generate the acoustic logs) with the depths at which the acoustic waves are generated, particularly when the downhole tools suffer from a phenomenon known as stick-slip. Certain known designs of transmitter address this aspect.
These known designs of dipole acoustic transmitter however are of limited benefit because the orthogonally directed pair of dipole pressure patterns they generate emanate at fixed orientations relative to the borehole.
This is often a disadvantage because it is difficult to control the orientation of the logging tool relative to the borehole. If the logging tool is orientated such that (for example) the pressure dipoles do not impinge along the principal axes of the borehole, excitation of dipole modes may diminish at the expense of unwanted excitation of a monopole (Stoneley) mode. In such circumstances even if the logging engineer is aware of a problem it could be completely impossible to take remedial steps. As a result valuable logging time could be wasted.
This can be a noticeable problem if the logging tool is “eccentered” in a borehole. In this condition the longitudinal central axis of the logging tool fails to coincide with the central longitudinal axis of the borehole. This is sometimes the case when the borehole extends horizontally, or substantially horizontally. In such a situation the logging tool tends to settle off-centre on the low side of the borehole. As a result the tool axis and the borehole axis in general are not coincident.
Eccentering of a logging tool can also arise in boreholes that do not extend predominantly horizontally; but it is encountered most commonly, and is hardest to remedy through attempting to reposition the logging tool, in substantially horizontal boreholes.
Eccentering is illustrated in FIG. 2a, which shows in transverse cross sectional view a horizontal borehole 16 in which a logging tool 10 has settled on the low side 17, opposite the high side 18, of the borehole.
In such a situation in order to minimize the effects of the eccentering problem ideally the poles X, Y of the dipoles should align respectively along the diameter 19 of the borehole containing the logging tool and along a chord 21 orthogonal thereto, in order to maximize the flexural mode excitation in the formation. As noted however in practice this can be hard or even impossible to achieve.
This is typically because the orientation of the tool 10 relative to the borehole means that the dipole axes X, Y are rarely aligned with the diameter and chord as desired. It moreover is not often possible to rotate the logging tool in the borehole in order to achieve the preferred orientation described above. This mis-alignment of the dipole axes is shown in FIG. 2a. 
A further characteristic of existing acoustic transmitters as known in the prior art is that when used to generate monopole pressure patterns their usefulness is limited. This is because the known designs based on piezoelectric hoops are not capable of generating monopole frequencies with significant energy below the approximate frequency range 12 KHz-25 KHz. This is particularly the case when the tool diameter is small. It is however desirable to generate lower monopole frequencies in some situations as described further herein.
U.S. Pat. No. 5,081,391 discloses an acoustic transducer in which four piezoelectric elements are bonded to four quadrants defined on the interior of a flexible cylindrical shell, such that opposed pairs of the piezoelectric elements lie on one or other of two mutually orthogonal axes extending transversely of the shell. The publication discloses driving the piezoelectric elements in order to generate dipole pressure patterns in a surrounding fluid.
In one embodiment U.S. Pat. No. 5,081,391 describes multiple piezoelectric elements bonded at regular intervals about the interior of the cylindrical shell. By selectively driving the elements multi-pole pressure patterns can be generated orientated at a plurality of fixed locations about the cylinder. The fact that the piezoelectric elements are bonded to the cylindrical shell however is stated in U.S. Pat. No. 5,081,391 adversely to influence efficiency of the transducer in this mode.
“A New Generation Crossed Dipole Logging Tool: Design and Case Histories”, Kessler et al, SPWLA 42nd Annual Logging Symposium, Jun. 17-20 2001, discloses a monopole and two dipole piezoelectric acoustic transmitters in a transducer body. The firing sequence of the transmitters is programmable.
WO 2010/091160 A2 discloses another cylindrical quadrant piezoelectric transducer design in which pressure pattern axes are generated a fixed orientations relative to the cylinder.
Further publications pertaining generally to acoustic transmitter design include U.S. Pat. Nos. 7,207,397, 7,460,435, 5,477,101, 4,525,645, 7,364,007, 5,109,698, 6,614,360 and 8,199,609.